The invention relates to a device for use in activating an explosive charge downhole in a wellbore.
After a wellbore has been cased and the casing cemented into place, it is common practice to form perforations in the casing and cement in the region of hydrocarbon bearing formation using a perforating gun. This is lowered into position, and then fired.
One prior art means of firing the gun is to use a firing head which is actuated by a metal bar dropped down the tubing string. While this means has the advantage that the gun does not go off prematurely, it obviously cannot be used in wellbores that deviate significantly from the vertical. Another disadvantage is that the firing head may become fouled by heavy drilling mud or debris before the bar is dropped, preventing actuation of the gun.
Various means have been proposed to use pressure applied at the wellhead to actuate the perforating gun. Because of the potentially serious consequences of premature detonation, the actuating mechanism must be able to prevent accidental detonation by pressure changes while the gun is being put into position.
One method is to apply pressure through the tubing string to a pressure responsive firing device, an example of which is disclosed in U.S. Pat. No. 4,544,034 to George. The firing device is in fluid communication with the surface, either using a liquid or nitrogen, and pressure applied at the surface is communicated through the fluid column to the firing device. While this is a useful system in many applications, it is not suitable in those circumstances where a full column of fluid in the tubing string cannot be used. It may also be unsuitable when the formation pressure is low, and it is not possible to bleed off sufficient fluid between actuation and firing to produce a condition of underbalance at the time of perforation.
A method of using the annulus pressure to activate the firing mechanism is disclosed in U.S. Pat. Nos. 4,484,632 to Vann and 4,564,076 to Vann et al. When the perforating gun is in position a packer is set above the formation. An increase in the annulus pressure above the packer is communicated to a lever mechanism inside the tube. At a predetermined pressure, shear pins break and the lever operates the firing mechanism of the perforating gun. This method permits shooting underbalanced, but has the disadvantage that the lever mechanism obstructs part of the tube bore, so that full bore tools cannot be passed down the tube beyond the firing device.